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Focussed ultrasonic emulsifiers and dispersers
Focussed
ultrasonic
emulsifiers
and dispersers
J. LAST* through a liquid phase by using curved elements elements have been in use for research purposes for some time. Radial mode transducers clamped tubing, which liquid to be processed is have also been made and are sold commercially to-day.
Brownian motion in the very small oil particles.
0.15HP. h/gal.
This can be compared colloid mill emulsions to ultrasonic)
Section of the Research Foundation focusses longitudinal ultrasonic waves internally carrier liquid
a
comDirectrix
which encloses 0 e
EMULSIFIER DEVELOPMENT Essentially, the design consists of a simple internal reflection of sound waves from a solid/gas parabolic interface. The reflections are brought to a focus inside the horn and the focus is contained by a cavity or channel (Fig 1). The driving element for the quarter-wave stub may be either electrostrictive or magnetostrictive. If the former is used it has been found that a sandwich construction using two quarter-inch thick doughnut PZT ceramic elements bolted to a stub through the centre is efficient, provided the frequency is kept in the low range (<35 kHz) and the fundamental wave-length is large compared to the thickness of the ceramic. The effective acoustic path length, that is, the distance between the driving element and the directrix of the parabola, is one quarter wave-length and the stub acts as a normal quarter-wave flat stub. A backing must be placed the other side of the driving element so that the entire unit behaves in half -wave resonance. Impedance matching of the crystal to the liquid is considered important and the selection of materials of construction is critical. In some cases the parabolic end may be placed at the end of a half-wave horn and a flat quarter-wave impedance transformer inserted between the half-wave and the driving element. High Q units are thus obtained with good impedance matching.
waves
I
face
Fig 1 The reflections of sound waves at a solid/gas parabolic interface are brought to a focus contained by a cavity or channel irectrix
Fig 2 Short focus parabolas (a) are preferred to long focus parabolas (b) because they minimize the interference effect of transverse reflected waves
It has been found preferable to use short focus parabolas (Fig 2) rather than those with a long focus in order to minimize the interference effect of transverse reflected waves which are also propagated on reflection from the metal/air interface. A double-ended short focus emulsifier horn may be seen in Fig 3. This unit, with an overall length of 19$ inches, is in resonance at 4.7 KHz, however its most efficient operating frequency is its seventh harmonic or 32.9 KHz. The ceramic transducer and attachments are contained in the large chamber seen in the centre. Cooled dielectric oil is constantly circulated through the chamber. The parabola is in one plane which presents a line focus and which is encompassed by the channel conducting the liquid to be emulsified, The liquid may be run through one channel, then through the other in order to get a double exposure to the high intensity ultrasound. Measurement of cavitation is difficult even in simple systems. Two techniques have been used with these parabolic emulsifiers: (1) the measurement of heat increase in a liquid passed through the channel and (2) the measurement of emulsion particle size. Typical results for the double ended horn in Fig 3 show approximately 100 acoustic watts in each end. The generator in this case supplied 300 watts. Fig 4 shows a typical vegetable oil/water emulsion mixed by a high speed counter rotating propellor and Fig 5 shows the same emulsion after one pass through the emulsifier. * Department of Engineering and Metallurgy, Ontario Research Foundation, Sheridan Park, Ontario, Canada
cm Fig 3 Double-ended short focus emulsifier horn resonant at 4.7 kHz, but operating at 32.9 kBz, the seventh harmonic
Fig 4 Typical vegetable oil/water emulsion mixed by a high speed counter rotating propellor Fig 5 Emulsion shown in Fig 4 after one pass through the emulsifier
ULTRASONICS April 1969
131
I I 1
ultrasonic
emulsifiers
and dispersers
J. LAST* through a liquid phase by using curved elements elements have been in use for research purposes for some time. Radial mode transducers clamped tubing, which liquid to be processed is have also been made and are sold commercially to-day.
Brownian motion in the very small oil particles.
0.15HP. h/gal.
This can be compared colloid mill emulsions to ultrasonic)
Section of the Research Foundation focusses longitudinal ultrasonic waves internally carrier liquid
a
comDirectrix
which encloses 0 e
EMULSIFIER DEVELOPMENT Essentially, the design consists of a simple internal reflection of sound waves from a solid/gas parabolic interface. The reflections are brought to a focus inside the horn and the focus is contained by a cavity or channel (Fig 1). The driving element for the quarter-wave stub may be either electrostrictive or magnetostrictive. If the former is used it has been found that a sandwich construction using two quarter-inch thick doughnut PZT ceramic elements bolted to a stub through the centre is efficient, provided the frequency is kept in the low range (<35 kHz) and the fundamental wave-length is large compared to the thickness of the ceramic. The effective acoustic path length, that is, the distance between the driving element and the directrix of the parabola, is one quarter wave-length and the stub acts as a normal quarter-wave flat stub. A backing must be placed the other side of the driving element so that the entire unit behaves in half -wave resonance. Impedance matching of the crystal to the liquid is considered important and the selection of materials of construction is critical. In some cases the parabolic end may be placed at the end of a half-wave horn and a flat quarter-wave impedance transformer inserted between the half-wave and the driving element. High Q units are thus obtained with good impedance matching.
waves
I
face
Fig 1 The reflections of sound waves at a solid/gas parabolic interface are brought to a focus contained by a cavity or channel irectrix
Fig 2 Short focus parabolas (a) are preferred to long focus parabolas (b) because they minimize the interference effect of transverse reflected waves
It has been found preferable to use short focus parabolas (Fig 2) rather than those with a long focus in order to minimize the interference effect of transverse reflected waves which are also propagated on reflection from the metal/air interface. A double-ended short focus emulsifier horn may be seen in Fig 3. This unit, with an overall length of 19$ inches, is in resonance at 4.7 KHz, however its most efficient operating frequency is its seventh harmonic or 32.9 KHz. The ceramic transducer and attachments are contained in the large chamber seen in the centre. Cooled dielectric oil is constantly circulated through the chamber. The parabola is in one plane which presents a line focus and which is encompassed by the channel conducting the liquid to be emulsified, The liquid may be run through one channel, then through the other in order to get a double exposure to the high intensity ultrasound. Measurement of cavitation is difficult even in simple systems. Two techniques have been used with these parabolic emulsifiers: (1) the measurement of heat increase in a liquid passed through the channel and (2) the measurement of emulsion particle size. Typical results for the double ended horn in Fig 3 show approximately 100 acoustic watts in each end. The generator in this case supplied 300 watts. Fig 4 shows a typical vegetable oil/water emulsion mixed by a high speed counter rotating propellor and Fig 5 shows the same emulsion after one pass through the emulsifier. * Department of Engineering and Metallurgy, Ontario Research Foundation, Sheridan Park, Ontario, Canada
cm Fig 3 Double-ended short focus emulsifier horn resonant at 4.7 kHz, but operating at 32.9 kBz, the seventh harmonic
Fig 4 Typical vegetable oil/water emulsion mixed by a high speed counter rotating propellor Fig 5 Emulsion shown in Fig 4 after one pass through the emulsifier
ULTRASONICS April 1969
131
I I 1